1. Field of Invention
The invention relates to a process for optical alignment in which alignment layers of a liquid crystal display (LCD) element and an angle of view field compensation film which is installed in a liquid crystal cell are irradiated with polarized light, and in which optical alignment is performed. The invention relates especially to a process for optical alignment in which the alignment layers can be aligned in the desired direction even when optical alignment is being performed with deviations of the polarization axis for the polarized light which is radiated onto the alignment layers.
2. Description of Related Art
A liquid crystal display element is produced as follows:                An alignment layer which is formed on the surface of a transparent substrate is subjected to treatment (alignment treatment) in which liquid crystals are aligned in the desired direction.        Two transparent substrates are placed on top of one another such that the alignment layers come to rest on the inside.        Liquid crystals are added between the alignment layers.        
With respect to alignment treatment of the alignment layers of the above described liquid crystal display element, there is a technique which is called optical alignment in which the alignment layers are irradiated and exposed with polarized light with a given wavelength and in which this alignment is thus carried out. The device for irradiation with polarized light for optical alignment can be one of the devices which are described, for example, in Japanese patent specification JP 2928226 B and Japanese patent specification JP 2960392 B (both of which correspond to U.S. Pat. No. 6,190,016 B1).
Recently, the above described device for irradiation with polarized light has been used more and more often, not only to produce the above described liquid crystal display element, but also to produce an angle of view field compensation film, also called an optical compensation film or a wide-view film. The angle of view field compensation film was produced by application of liquid crystals for UV curing to a base film, alignment of the liquid crystal molecules in a certain direction, afterwards curing the liquid crystals by UV irradiation, and thus, fixing the direction of the liquid crystal molecules. By cementing an angle of view field compensation film to a liquid crystal display a reduction of the image quality is compensated. The film for producing optical alignment including the angle of view field compensation film is called an “optical alignment layer” below.
In the devices for irradiation with polarized light for optical alignment of the alignment layers of a liquid crystal display element which were described in the above described Japanese patents, as is also shown in FIG. 7 of JP 2928226 B and FIG. 5 in JP 2960392 B (FIG. 9 of corresponding U.S. Pat. No. 6,190,016 B1), a workpiece W which is a substrate, an angle of view field compensation film or the like, onto which an optical alignment layer has been applied, is irradiated with polarized exit light. For this purpose, UV-containing light which has been emitted from a light irradiation source is allowed to be incident in a polarization element in which, for example, several glass plates inclined at the Brewster angle, and is thus subjected to polarization separation. Then, the light is routed via an integrator lens, a plane mirror, a collimation lens, a collimation mirror or the like.
For optical alignment of an optical alignment layer, polarized light with a given wavelength (UV radiation, for example, with a wavelength of 280 nm to 320 nm) and with an extinction ratio at least equal to a given value (for example, with a ratio of the proportion of S-polarized light to P-polarized light of 1/10 to 1/100) is necessary. This is determined by the physical properties of the above described optical alignment layer. The term “extinction ratio” is defined as the ratio of the proportion of P-polarized light to the proportion of S-polarized light which are contained in the polarized light.
Recently, as the parameter for executing optical alignment, in addition to the above described wavelength and extinction ratio, also the divergence of the direction of the polarized light (hereinafter called the “polarization axis”) within the irradiation surface has been considered more and more.
When using the device for irradiation with polarized light in the above described conventional example, the divergence within the surface of the polarization axis on the irradiation area is roughly ±0.5°, as is shown, for example, in FIG. 5. When deviations of the polarization axis occur in the polarized light with which the alignment layer is irradiated, divergence of the alignment direction of the alignment layer is caused; this can lead to faulty parts.
This means that, when optical alignment is being performed with light having large divergence within the surface of the polarization axis, the contrast of the liquid crystal display element as a finished part differs from place to place; this causes a reduction of image quality.
To eliminate divergence of the polarization axis approaches are suggested in Japanese Patent Publication JP2004347668 and Japanese Patent Publication JP2005128271. However, in the approaches of these publications, there is the disadvantage that there needs to be a special arrangement in the light irradiation means for eliminating the divergence of the polarization axis and that increases costs.
Furthermore, as a light source for irradiation with polarization light, also the use of a rod-shaped lamp has been suggested which is more easily used for irradiating a large area than a point light source lamp according to the above described conventional example. However, using a rod-shaped lamp makes it difficult to eliminate divergence of the polarization axis.